Context: We are working on migration of the driver, which is currently represented as a kernel extension, to the DriverKit framework. The driver works with Thunderbolt RAID storage devices. When connected through the Thunderbolt interface to the host, the device itself presents in the OS as a PCI device. The main function of our driver (.kext) is to create a "virtual" SCSI device in the OS for each virtual RAID array. So that the OS can work with these SCSI drives as usual disk storage.  We use https://developer.apple.com/documentation/scsicontrollerdriverkit to migrate this functionality in the dext version of the driver. Current issue: When a device is connected - the dext driver cannot create a SCSI drive in the OS. Technically our dext tries to create a SCSI drive using the UserCreateTargetForID method. On this step the OS sends the first SCSI command “Test Unit Ready ''to the device to check if it is a SCSI device. We process this command in an additional thread  separated from the main process of the dext (as it is recommended in the DriverKit documentation). We can see in the logs that the device receives this command and responses but when our dext sends this response to the OS the process is stuck in the waiting mode. How can we understand why it happens and fix it? More details: We are migrating functionality of an already existing “.kext” driver. We checked logs of the kext driver of this step: 15:06:17.902539+0700 Target device try to create for idx:0 15:06:17.902704+0700 Send command 0 for target 0 len 0 15:06:18.161777+0700 Complete command: 0 for target: 0 Len: 0 status: 0 flags: 0 15:06:18.161884+0700 Send command 18 for target 0 len 6 15:06:18.161956+0700 Complete command: 18 for target: 0 Len: 6 status: 0 flags: 0 15:06:18.162010+0700 Send command 18 for target 0 len 44 15:06:18.172972+0700 Complete command: 18 for target: 0 Len: 44 status: 0 flags: 0 15:06:18.275501+0700 Send command 18 for target 0 len 36 15:06:18.275584+0700 Complete command: 18 for target: 0 Len: 36 status: 0 flags: 0 15:06:18.276257+0700 Target device created for idx:0 We can see a successful message “Target device created for idx:0” In the the dext logs of the same step: We do not see the “Send command 18 for target 0 len 6” as we have in the kext logs  no log of the successful result “Target device created for idx:0” 15:54:10.903466+0700 Try to create target for 0 UUID 432421434863538456 15:54:10.903633+0700 UserDoesHBAPerformAutoSense 15:54:10.903763+0700 UserInitializeTargetForID 15:54:10.903876+0700 UserDoesHBASupportMultiPathing 15:54:10.904200+0700 UserProcessParallelTask start. 15:54:10.904298+0700 Sent command : 0 len 0 for target 0 15:54:11.163003+0700 Disable interrupts. 15:54:11.163077+0700 Complete cmd : 0 for target: 0 len: 0  status: 0 flags: 0 15:54:11.163085+0700 Enable interrupts. Will appreciate any help